This invention relates to recovery of oil from a subterranean reservoir through the use of a surfactant-cosurfactant system.
It has long been known that the primary recovery of oil from a subterranean formation leaves a substantial amount of the initial oil still in the formation. This has led to the use of what is commonly referred to as secondary recovery or water flooding wherein a fluid such as brine is injected into a well to force the oil from the pores of the reservoir toward a recovery well. However, this technique also leaves substantial amounts of oil in the reservoir because of the capillary retention of the oil. Accordingly, it has been suggested to use surfactants in waterflooding processes. It has been found that the use of surfactants can reduce the interfacial tension between the oil and the water to such an extent that substantially increased quantities of oil can be displaced. These surfactants have been used in both systems forming microemulsions and those not forming microemulsions.
The microemulsions which have been proposed have been selected from compositions in the single phase region of a ternary diagram. Such microemulsion systems can be either oil-external microemulsions or water-external microemulsions. When such microemulsion systems are used, it is viewed that the initial stages of oil recovery involve an efficient miscible displacement with subsequent involvement of immiscible displacement upon a breaking down of the microemulsion into multiphases upon dilution of the microemulsion with crude oil and resident water at its leading edge and dilution with an aqueous drive fluid at its trailing edge. Hence, optimization of such microemulsion surfactant flood systems is approached in terms of minimization of the multiphase region in the phase diagram so as to prolong miscible displacement, with low interfacial tensions in the multiphase regions so as to enhance immiscible displacement. From a practical standpoint, however, the development of effective microemulsion systems which can economically recover the crude oil from a subterranean formation suffers from certain drawbacks in that it is difficult to maintain the miscible displacement and it is difficult to obtain thereafter the low interfacial tensions necessary to provide effective immiscible displacement of the crude oil.
Surfactant systems have been developed which form microemulsions on contact with oil. One of the early patents describing such a technique is Cooke, Jr., U.S. Pat. No. 3,373,809 (Mar. 19, 1968) which discloses recovering oil through the formation of a microemulsion formed in situ by injecting a surfactant system. Cooke, Jr., took the approach that the effectiveness of such microemulsions was based on the formation of a single phase microemulsion with the formation oil and hence gave rather explicit instructions as to the technique to achieve the results desired, namely, the injection of a high concentration of surfactant. All systems discussed above can be quite effective because they establish a miscible condition, but the cost of the large quantity of surfactant required, which usually is in excess of about 7-15% by weight so as to provide a composition within the single phase region of the ternary diagram, can easily exceed the value of the oil recovered. It is becoming well recognized that it is impractical from an economic standpoint to maintain in a reservoir formation such a highly concentrated surfactant composition which remains effectively miscible (i.e., in the single phase region) throughout the lifetime of the surfactant flood as proposed by Cooke and others.
Recent work has led to the suggestion of injecting microemulsion systems wherein the microemulsion phase is immiscible with the resident fluids in the reservoir. Reed et al, U.S. Pat. No. 3,885,628 (May 27, 1975) propose to form a multiphase microemulsion system above ground by mixing oil, brine and surfactant and injecting at least the immiscible microemulsion phase. In some cases Reed et al suggest injecting one or more of the other phases which exist in equilibrium with the microemulsion phase along with the immiscible microemulsion phase. Later, work along this line is exemplified by Healy, U.S. Pat. No. 3,981,361 (Sept. 21, 1976) which describes procedures for producing surfactant systems above ground which are injected as an immiscible microemulsion. In this later case, emphasis is placed on injection of the single immiscible surfactant-rich microemulsion phase. Ossip et al, U.S. Pat. No. 3,938,591 (Feb. 17, 1976) also discusses the injection of immiscible microemulsion systems which as with Reed et al and Healy resist uptake of oil and water into the immiscible microemulsion phase. In all of these cases, the disclosed procedure has the obvious disadvantage of requiring the injection of a composition containing substantial amounts of oil which adds to the cost of the injected composition. In addition, there is the problem of achieving the optimum system for a given oil since it turns out that different oils behave differently.
Nowhere in the art is there any teaching of the use of an optimum aqueous surfactant system which is capable of forming a multiphase microemulsion system in situ, wherein said multiphase microemulsion system is highly effective in displacing the particular oil of interest.